This application relates to a measuring method or arrangement which permits the quick, accurate, and safe determination of the most important electric parameters of binders which are used for the electrocoating process. These parameters include the electrochemical deposition equivalent (or Coulombic yield); the measurement of the peak current occurring at the beginning of the deposition, and the measurement of the electric energy used for coating.
As used herein, the electrochemical deposition equivalent is the charge volume which must be used in order to deposit one milligram of a binder (Cb/mg). The reciprocal value is designated as the Coulombic yield. From this value, the efficiency of the binder deposition can be determined, as well as the degree of neutralization of the resin. Moreover, the measurement can be used for production control, i.e., to determine deviating coating characteristics, for example, a higher or lower acid number in the composition or the presence of foreign electrolytes. The knowledge of the peak current which occurs at the beginning of coating operation, is of decisive importance for the comparison of ELECTROCOATING (EC) resins under variable conditions, or it can serve for the determination of the critical current density at which a rupture of the film occurs during coating. The peak current is also important in determining the requirements of the electrical installations, including the size of rectifier elements, thyristors, and the surge current stress of electric installations such as laboratory rectifiers, etc. The knowledge of the electric energy needed for the coating of an article permits the calculation of the current costs, as well as the costs for the installations of an electrodeposition bath (ETL), such as transformers and cooling installations.
The Coulomb measurement, i.e., the determination by measurement techniques of the flowed charge volume, hitherto had been determined with difficulties. In the case of coatings under a constant voltage, a peak current occurs at the beginning of the deposition, and afterwards a quick reduction of current occurs as a result of the build-up of the insulating paint film; whereby one can almost speak of an exponential drop of the current flow. The classical method of Coulomb measurement with the aid of a coulometer cannot be employed since the deposition of silver in the case of the high currents which occur (up to 20 A in the case of surfaces of 200 cm.sup.2 that are to be coated) cannot be accurately determined with known measuring techniques, and the utilization of the measuring results is cumbersome and time consuming.
As a further measuring method the X--Y-recorder which records the change of the current on a continuous paper tape was suggested. With this recorder, however, it turned out that the mechanical recording of the current/time curve is too slow, so that the first deposition phase is determined only imprecisely. Moreover, it was necessary to cut out the curve and weigh the weight of the paper in comparison with the weight of the paper of known Coulomb quantities to determine the number of the Coulombs which had flowed. This resulted in an error in measurement up to about 10%.
In the case of integrating recorders, the cumbersome cutting and weighing of tape was eliminated. However, the mechanical inertia of the writing system still had to be taken into consideration, and was still a source for error inasmuch as such recorders require at least one second in order to achieve their maximum deflection.
Another frequently used method for determining the number of the Coulomb is by deposition under a constant current intensity. This method has the disadvantage in that the build up of the film is accomplished under basically different prerequisites. Moreover, the duration of measurement is limited in time, since with the development of the insulating paint film the deposition voltage increases until, upon exceeding a certain voltage, the rupture of the film occurs and after the no defined deposition takes place.
The measurement of the peak current occurring at the beginning of the coating provides particular difficulties, since after only a short time the flow of current is reduced very considerably due to the resistance of the film which builds up. Values obtained by experience show that in the case of the usual voltages, the deposition current after 2-3 sec. of coating will reach only half of its starting value. A precise measurement of this peak current could only be done before this with the help of a storage oscillograph which alone is in a position to reproduce one-time processes such as depositions of electrocoating (ETL) binders ("Elektrotauchlackierung") on the picture screen. The recording of the change of the current with the electron ray of an oscillograph tube is accomplished almost without inertia. However, working with such an oscillograph is very expensive and time consuming and requires experienced personnel. Apart from this, measurements of oscillographic peak current can be carried out only with a stabilized DC voltage, since the half waves of the rectified AC voltage of a rectifier used customarily for the electrodeposition (ETL coating) are almost of the same order of magnitude as the duration of the peak voltage itself (width of the half WAVE 10 msec. duration of peak current 50-100 msec.). Energy that is to be used for coating can be calculated from the deposition voltage and from the Coulomb quantity that has flowed during this time. For the case of a coating under a constant DC voltage there are no problems. However, such voltage is almost never available, since for the high voltages and currents which occur during the electro-(ETL) coating, the apparatuses necessary for this purpose are far too costly. In addition, under such conditions is too far removed from practice to be of value. The rectifier installations which are in use, in part have inner resistances which cannot be neglected, and in addition voltage and current distortions occur which cannot be measured with volt and ammeter, whenever the electric control is accomplished via a thyristor control. In practice, therefore, the elastic energy which is used up in the case of coating can be determined only approximately.